A new and potentially most useful method for diagnoses and therapy of metastasized cancer has been developed. We have succeeded in attaching metal chelates containing radionuclides to tumor cell specific monoclonal antibodies in such a way that neither the activity nor specificty of the antibody is adversely affected. These labelled antibodies have been shown to localize specifically in tumor tissues in vivo. In vitro assays reveal that antibody-antigen binding occurs on the cell specifically. Methods for incorporating isotopes suitable for therapy of tumor cells by use of radiometal chelate conjugated antibodies have also been developed.